Add shop, POS sync, event ticketing, and shipping catalog apps (#6)

## Summary
- Closes #5
- Optional `shop`, `pos_sync`, `events`, and `shipping` apps gated by `FEATURE_*` flags
- Deps match the catalog: shop/events need email + Stripe; POS/shipping need shop
- Portal inventory, POS webhooks, capacity tickets, EasyPost/Pirate Ship shipping

## Test plan
- [ ] `manage.py test` (152 passed locally)
- [ ] Shop cart + paid order decrements stocked inventory
- [ ] POS inbound webhook decrements SKU; paid order queues outbound reserve
- [ ] Event capacity blocks overbook; paid order emails ticket codes
- [ ] Shipping stub label + Pirate Ship CSV of unshipped paid orders
- [ ] `validate-env.sh` rejects shop without email/payments, POS/shipping without shop

Reviewed-on: #6

site/shipping/admin.py

@@ -0,0 +1,10 @@
+from django.contrib import admin
+
+from shipping.models import Shipment
+
+
+@admin.register(Shipment)
+class ShipmentAdmin(admin.ModelAdmin):
+    list_display = ("order", "status", "carrier", "tracking_number")
+    list_filter = ("status", "carrier")
+    search_fields = ("tracking_number", "order__number")